Selective effects of phosphodiesterase inhibitors on different phosphodiesterases, adenosine 3',5'-monophosphate metabolism, and lipolysis in 3T3-L1 adipocytes.

3T3-L1 adipocytes contain both soluble and particulate cAMP phosphodiesterases which can be distinguished by several criteria. Particulate phosphodiesterase activity of 3T3-L1 adipocytes, but not undifferentiated fibroblasts, was selectively increased by incubation of cells with insulin or lipolytic hormones. Particulate cAMP phosphodiesterase activity from 3T3-L1 adipocytes was very sensitive to inhibition by cilostamide in an apparently competitive fashion. Particulate activity from undifferentiated 3T3-L1 fibroblasts or supernatant activity from either type of cell was much less sensitive to cilostamide. On the other hand, supernatant cAMP phosphodiesterase activity from both undifferentiated fibroblasts and 3T3-L1 adipocytes was very sensitive to inhibition by Ro-20-1724 in an apparently competitive fashion. Ro-20-1724 was not an effective inhibitor of particulate activity from either type of cell. In fractions from 3T3-L1 adipocytes, isobutylmethylxanthine (IBMX) effectively inhibited both supernatant and particulate cAMP phosphodiesterase activities. In addition, however, IBMX was relatively more specific in inhibiting supernatant calmodulin-activated cGMP phosphodiesterase activity than supernatant calmodulin-independent or particulate cGMP phosphodiesterase activities. In intact 3T3-L1 adipocytes, cilostamide enhanced lipolysis in the absence or presence of isoproterenol and had no effect on cAMP content in the presence of low concentrations of isoproterenol. Ro-20-1724 increased lipolysis to a lesser extent than cilostamide and did not enhance isoproterenol-stimulated lipolysis, but did increase isoproterenol-stimulated accumulation of cAMP to a greater extent than cilostamide. Like cilostamide, Ro-20-1724 did not enhance accumulation of cAMP in the absence of isoproterenol. IBMX enhanced lipolysis and cAMP accumulation with or without isoproterenol. Taken together, these results support the idea that although particulate and soluble low Km phosphodiesterases influence cAMP content, the particulate enzyme may be more important in the metabolism of cAMP involved in the regulation of lipolysis. Since combinations of Ro-20-1724 and cilostamide were not as effective as IBMX in increasing cAMP content, perhaps the calmodulin-dependent phosphodiesterase, which is selectively inhibited by IBMX, is also involved in the regulation of total cell cAMP content.